All the ampules, found in a storeroom between wards A1 and A3, had been injected with insulin.

Leighton, who works on A3, was taken into custody after detectives swooped on the first-floor flat she shares with 28-year-old fiancé Tim Papworth, above a darts equipment shop a mile from the hospital.

Ten crime scene investigators then moved in to search the apartment where the couple have lived for the past six months.

Shocked neighbours said Leighton was “the last person on Earth” who could have been involved in the killings.

One said: “She’s just a lovely girl. She’s a nurse and lives to save lives.

“This just doesn’t add up. She simply cannot be involved in anything like this.’’

Hamid Bayatboor, 47, who regularly served Leighton in his convenience store, said: “She was a really nice girl, always laughing.

“When I told her my daughter wanted to be a doctor she gave me some advice to pass on to her about how she should apply.

“She talked like she thought she had the best job in the world. I am completely shocked she is the person who has been arrested,” he added.

“I can only imagine it’s some kind of mistake.’’

Another shopkeeper said he was “stunned” when six “burly cops” leapt out of a Toyota pickup and marched into the darts store and up into the flat.

They were joined by four more officers carrying rucksacks 10 minutes later.

The team spent up to eight hours inside the apartment while the shop’s shutters stayed down.

Open University graduate Leighton describes herself on her Facebook page as a “happy go lucky kinda gal’’ who “loves the wkend and having a laugh with the people that I call friends for a reason”.

Last night no-one answered the door at the home of her parents David, 51, and Lyn, 55, who trains nurses at Stepping Hill.

Police sources told the Daily Star the investigation was still “active” and they have not ruled out further arrests.

Nurses have been told to work in pairs and hospital security has been increased.

Staff first raised the alarm after grandmother Tracey Arden, 44, who had been admitted with an illness related to her multiple sclerosis, died suddenly on July 7 after apparently appearing to make a recovery.

Retired businessman Mr Keep, 84, who had broken a hip, was described by friends as “very strong for his age”, but also deteriorated too quickly for doctors to save him.